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Researchers from NASA and the US National Snow and Ice Data Centre (NSIDC) have warned that the arctic ice cap could completely disappear within a century, after a satellite survey this summer revealed ice cover was at its lowest level ever.

Sea ice coverage was just 2.06m square miles, the scientists said, which is around 20 per cent below the average cover at this time of year in the 1970s. This is low enough to put many arctic species, including the polar bear, at risk.

The scientists said that the reduction in ice cover - equivalent to an area twice the size of Texas (or 62 times the size of Wales) was "stunning". It is also the fourth consecutive year in which cover has fallen.

NSIDC's Walt Meier commented: "Having four years in a row with such low ice extents has never been seen before in the satellite record. It clearly indicates a downward trend, not just a short-term anomaly."

The sea ice in the arctic always retreats in summer, but generally builds back up again during the colder winter months. Last year, however, the ice did not approach its normal winter coverage, so when the spring thaw came, levels were already low.

However, as always with news of this sort, there are some voices calling for a stay of judgment until all the facts are in. The earliest satellite records available for comparison are those from 1978. Some scientists are cautious about making long term trend predictions based on relatively short term data.

Others argue that the Arctic is particularly vulnerable to global warming, because small temperature changes quickly become a positive feedback loop. When ice melts, more ground or water is exposed, both of which absorb more solar energy than ice or snow.

Peter Bond from the Royal Astronomical Society says that data from the CryoSat mission, scheduled for launch on 8 October, should help settle the argument. ®